1. Field of the Invention
The present invention relates to a method of manufacturing a corona discharge device which may suitably be used as an ozonizer for deodorization of ambient air bearing malodorous substances.
2. Description of the Prior Art
To provide ease and comfort of living spaces, such as lavatories, wherein sources of offensive smell are present, various deodorization techniques have been used to remove unfavorable smell from the ambient atmosphere.
Conventional deodorization techniques typically include the use of ozone generators which are commonly referred-to in the art as ozonizers. In many instances, an ozonizer is preferably used in combination with an ozone decomposer catalyst, as described, for example, in Japanese Utility Model Kokai Publication No. 1-128822(1989). More specifically, an ozonizer is placed in an air passage of a deodorizer equipment through which ambient air containing malodorous substances is circulated by a fan. A high frequency alternating voltage is applied to the ozonizer to develop corona discharge across an air gap thereof whereby ozone is artificially produced. The air-borne malodorants as well as ozone generated by the ozonizer are forced to contact with the ozone decomposer catalyst, such as TiO.sub.2 and MnO, carried by a porous catalyst support formed, for example, in a honeycomb structure. Conveniently, the extended surface area of the ozone decomposing catalyst serves to effectively capture the malodorants thereon. The catalyst functions to decompose ozone into oxygen molecules and active oxygen atoms. The active oxygen, in turn, reacts with the malodorants captured on the catalyst surface to oxidize them into odorless or less malodorous substances. As an example, when malodorant is H.sub.2 S, chemical reaction of deodorization takes place as follows. EQU H.sub.2 S+3O.sub.3 .fwdarw.SO.sub.2 +H.sub.2 O.degree.3O.sub.2
The flow of air treated in this manner by the deodorizer equipment is then discharged into the ambient atmosphere.
Since ozone is toxic and has a unique stimulative smell, it is desirable that treated air leaving the deodorizer apparatus be ozone free. In this regard, an important function of the conventional ozone decomposer catalyst is to decompose any excessive amount of ozone into harmless oxygen in the following manner. EQU 2O.sub.3 .fwdarw.3O.sub.2
However, it has been recognized that, because the amount of ozone decomposer catalyst filled in the currently available deodorizer equipment is limited due to the size thereof, the overall efficiency of the ozone decomposer catalyst generally does not exceed about 90%, with a result that about 10% of ozone produced by the ozonizer is unavoidably released into the atmosphere without being decomposed.
The present inventors have observed that about 0.02 ppm of ozone contained in a flow of air flowing at a flow rate of 100 liters per minute may be sensed by human respiratory organs. Assuming that the efficiency of the ozone decomposer catalyst is 90%, it follows that, desirably, an ozonizer should not generate ozone at a production rate greater than about as small as 0.2 ppm for air flowing at a flow rate of 100 liters per minute.
Another requirement for the deodorizing ozonizer is that such a limited ozone generating capability must be sustained for years in order to adequately deodorize the environment throughout the service life of the ozonizer.
In short, the problem which must be overcome in designing an ozonizer for deodorizer applications is to ensure that a controlled, small amount of ozone is generated constantly for a prolonged period of time.
Various types of the prior art ozonizers will be briefly discussed below. It is known in the art that the principle of operation of an ozonizer, wherein ozone is artificially produced, is found in the "Siemens Tube" developed in as early as 1857. This device is provided with a double-walled glass tube forming an annular air passage through which dry air or oxygen under atmospheric pressure is circulated. An inner electrode is provided at the center of the glass tube and an outer casing surrounding the tube serves as an outer electrode. When an alternating high voltage is applied between the inner and outer electrodes, silent electric discharge which is otherwise known as corona discharge is developed across the air passage whereby oxygen is converted into ozone. In the Siemens Tube, corona discharge is spread over the inner surfaces of the glass tube and this phenomenon is known in the art as surface creepage or surface flashover. It is said that such flashover is due to the presence of the glass tube which acts as an insulating barrier between electrodes to cause electric discharge columns, which in actuality are streams or avalanches of electrons, to be distributed over the surfaces of the glass tube.
Recent ozonizers are made generally by using manufacturing techniques of solid state devices and have one or more planar electrodes but operate on as much the same principle as the Siemens Tube.
For example, Japanese Patent Kokai Publication No. 61-231573(1983) and FIG. 2 of Japanese Patent Kokai Publication No. 60-157183(1985) describe corona discharge devices or ozonizers of the opposed electrode type, a cross-section of which is reproduced schematically in FIG. 1 of the drawings accompanying the present application. As shown therein, an inner planar electrode 10 is embedded in a substrate 12 of dielectric ceramic material and an outer electrode 14 is formed on the surface of the substrate by metallizing techniques such as tungsten paste printing. As a high frequency alternating voltage is applied between the electrodes by a power source 16, an electric field is developed across the dielectric layer. In FIG. 1, the direction of the electric field is shown by the lines of electric force indicated by the broken lines 18, the lines of electric force being perpendicular to the equipotential surfaces indicated by the fine solid lines 20. It will be noted that, because the outer electrode 14 is narrower than the inner electrode 10, the equipotential surfaces 20 are flared upwardly so as to cause part of the electric field to be developed across the air gap. When at any point of time the electric potential applied across the air gap exceeds the breakdown voltage thereof, breakdown of the air gap takes place thereby resulting in electric discharge occurring along the lines of electric force as schematically illustrated in FIG. 1 by the bold lines 22. Corona discharge is observed as a crowd of discharge columns of such individual discrete electric discharge occurring consecutively. Such corona discharge is spread to a certain extent over the surface of the dielectric substrate due to the surface creepage or flashover mentioned hereinbefore.
The disadvantage of the ozonizer of the opposed electrode type as reproduced in FIG. 1 is that it is difficult to provide a durable and reliable electrical connection to the outer electrode 14. Thus, one of the lead wires from the power source 16 must necessarily be arranged to extend above the outer electrode, with an end thereof soldered to the upper surface of the outer electrode. As the lead wire as well as the soldered end are arranged in this manner in an ozone enriched region, there is a risk that they are readily degraded due to oxidation. The outer electrode is also subjected to the attack by ozone.
Japanese Patent Kokai Publication Nos. 64-33004(1989) and 1-246104(1989) disclose an ozonizer of the opposed electrode type wherein the outer electrode is covered by a protective coating. While the ceramic coating protects the outer electrode from oxidation, this arrangement still suffers from the disadvantage of degradation of lead wire and soldered connection.
Japanese Patent Kokai Publication No. 58-108559(1983) and U.S. Pat. No. 4,783,716 describe a discharge device having a pair of inner planar electrodes juxtaposed in a side-by-side arrangement in a dielectric layer. The cross-sectional representation thereof is schematically reproduced in FIG. 2 hereof. Advantageously, this juxtaposed electrode arrangement enables to position all the lead wires at the lower side of the device remote from the ozone rich region. Therefore, the lead wires are exempt from chemical attack by ozone. It seems, however, that this discharge device has not been commercialized. Presumably, this is because of failure to produce an adequate amount of ozone. It is believed that, since the electric field developed between the juxtaposed electrodes 24 and 26 is mostly confined within the dielectric layer as illustrated in FIG. 2, it is difficult to develop across the air gap an electric field strong enough to generate intensive corona discharge.
Japanese Patent Kokai Publication No. 60-157183(1985) discloses in FIGS. 4-8 thereof a solid state discharge device having an additional floating electrode overlying a pair of juxtaposed inner electrodes to which an alternating voltage is applied. Similar device is disclosed in FIGS. 3-13 of Japanese Patent Kokai Publication No. 62-51463(1987) and FIGS. 1-4 of Japanese Patent Kokai Publication No. 3-190077(1991). For ready reference, a cross-sectional schematic view of these devices is reproduced in FIG. 3 of the present application. It will be understood from FIG. 3 hereof that the intermediate floating electrode 28 functions to capacitively couple the inner juxtaposed electrodes 30 and 32 with each other. Accordingly, as compared with the arrangement illustrated in FIG. 2, the presence of the floating electrode 28 contributes to raise the lines of electric force toward the air gap, thereby provoking more intensive corona discharge between the electrodes 28 and 30, in the one place, and between the electrodes 28 and 32, in the other place.
According to testings and investigations of the present inventors, however, it has been observed that the amount of ozone produced therein decreases rapidly as time elapses. The outer floating electrode 28 is readily oxidized because it is subjected not only to chemical attack by ozone which is generated but also to electrical attack by corona discharge. In addition, the floating electrode is damaged by sputtering due to ion bombardment. As a result, the electric resistance of the floating electrode becomes increased as the device is operated. Accordingly, the minimum voltage level necessary to trigger corona discharge becomes increased in response to the lapse of time, as described later in more detail with reference to the test results indicated in the accompanying drawings. This means that, when the ozonizer is to be operated at a predetermined operating voltage, the intensity of corona discharge will slide down so that the ozonizer will finally fail to produce required amount of ozone.
Accordingly, an object of the present invention is to provide an improved corona discharge device.
Another object of the invention is to provide a corona discharge device for use as an ozonizer, wherein ozone is generated at a constant rate for an extended period of time.
A still another object of the invention is to provide a corona discharge device which is capable of producing a controlled small amount of ozone throughout a prolonged service life.
A further object of the invention is to provide a method by which high quality corona discharge devices are manufactured on a mass production basis.